Account incorrectly billed

I have two contracts with mtn and I feel ****** by this company. I did an upgrade on the 5th of September and month end I paid the new installment of R279.. I did another upgrade on the 7th of October and month end I was billed R432 for a contract that I agreed to pay R199 based on my agreement with the consultant during the upgrade. When I signed the contract I saw that there was a mistake on the contract which I raised with the consultant and she told me its a system error which mtn is aware of. I call in to the call center then I was informed that there was an extra 2gb value service that was added to my account without my consent .What is the difference between the two contracts ? How do you charge Your pro rata? Your call centre can not explain anything to me.. I call to request my contract and I am told I have to go to the branch because I want to request a dispute for my billing, I hardly have time to go to the branch. I need clarity on this because I have already logged two complaints and no one is assisting me.